Longitudinal and transverse profiles of hilly and mountainous watersheds in Japan

Digital elevation models (DEMs) at 1-m resolution (for small bare lands; 0.18–0.43 km2) and at 10-m resolution (for broad forested areas; 28.9–100.5 km2) in Japanese hills and mountains were utilized for quantitative analyses of longitudinal and transverse profiles of watersheds, with special attention to changes in watershed form with regional relief. Statistical morphometric parameters representing the shape of these profiles were investigated. A previous study on a small steep watershed in Japan showed that the longitudinal and transverse characteristics of watersheds play different roles in determining the entire watershed topography, although some of the parameters representing the two characteristics are systematically related. This paper expands such analyses into various relief levels and larger spatial scales. For instance, average watershed slope in the high-relief areas mainly reflects the longitudinal slope, while the effects of the transverse slope become stronger in the lower-relief areas. Most topographic characteristics of watersheds in the high-relief areas are simpler and more organized than those in the low- to middle-relief areas, indicating that high-relief terrains have more mature conditions close to equilibrium. However, geomorphological complexity of watersheds does not always decrease with increasing relief, because differences in bedrock erodibility also affect watershed topography.